A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events. A so-called "VVI" pacemaker, for example, senses electrical cardiac activity in the ventricle of the patient's heart, and delivers pacing stimuli to the ventricle only in the absence of electrical signals indicative of natural ventricular contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the patient's heart, and delivers atrial pacing stimuli in the absence of signals indicative of natural atrial contractions, and ventricular pacing stimuli in the absence of signals indicative of natural ventricular contractions. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events.
Pacemakers are also known which respond to other types of physiologically-based signals, such as signals from sensors for measuring the pressure inside the patient's ventricle or for measuring the level of the patient's physical activity. In recent years, pacemakers which measure the metabolic demand for oxygen and vary the pacing rate in response thereto have become widely available. Perhaps the most popularly employed method for measuring the need for oxygenated blood is to measure the physical activity of the patient by means of a piezoelectric transducer. Such a pacemaker is disclosed in U.S. Pat. No. 4,485,813 issued to Anderson et al. Alternatively, oxygen saturation may be measured directly as disclosed in U.S. Pat. No. 4,467,807 issued to Bornzin, U.S. Pat. No. 4,807,629 issued to Baudino et al., and in U.S. Pat. No. 4,750,495 issued to Brumwell et al. Other parameters employed to measure the metabolic demand for oxygenated blood include right ventricular blood pressure and the change of right ventricular blood pressure over time, venous blood temperature, respiration rate, minute ventilation, and various pre- and post-systolic time intervals measured by impedance or pressure sensing within the right ventricle of the heart.
In typical prior art rate-responsive pacemakers, the pacing rate is determined according to the output from an activity sensor. The pacing rate is variable between a predetermined maximum and minimum level, which may be selectable by a physician from among a plurality of programmable upper and lower rate limit settings. When the activity sensor output indicates that the patient's activity level has increased, the pacing rate is increased accordingly. As long as patient activity continues to be indicated, the pacing rate is periodically increased by some incremental amount, until the programmed upper rate limit is reached. When patient activity ceases, the pacing rate is gradually reduced, until the programmed lower rate limit is reached.
A piezoelectric crystal is typically fixed to the pacemaker shield and generates an electrical signal in response to deflections of the pacemaker shield caused by patient activity. Piezoelectric, microphone-like sensors are widely used in rate-responsive pacemakers because they are relatively inexpensive, their manufactured yield is high, and they transduce the acoustic energy of patients' motion in a highly reliable manner.
In one prior art technique for employing a piezoelectric, microphone-like sensor for transducing patient activity, the raw electrical signal output from the sensor is applied to an AC-coupled system which bandpass filters the signal prior to being applied to pacemaker rate-setting logic. This arrangement is disclosed, for example, in U.S. Pat. No. 5,052,388 to Sivula et al., assigned by the assignee of the present invention and incorporated herein by reference. According to the Sivula et al. reference, peaks in the bandpass filtered sensor signal which exceed a predetermined threshold are interpreted by the rate-setting logic as an indication of patient activity of sufficient magnitude that an increase in the pacing rate may be warranted. The predetermined threshold, which may also be selectable by a physician from one of a plurality of programmable settings, is intended to screen out background "noise" in the sensor output signal indicative of low amplitude patient motion. Each occurrence of a peak in the bandpass-filtered sensor signal which exceeds the threshold level is known as an activity count. A sum of activity counts is computed over some period of time; for example, the number of activity counts may be determined every two seconds. If, at the end of that period, the number of activity counts exceeds some predetermined value, the ratesetting logic interprets this as an indication that the pacing rate should be incrementally increased.
In the utilization of sensor signals in the manner just described, for a given threshold setting, the rate-setting logic does not distinguish between different levels of physical activity; that is, each activity count is weighted equally, and the magnitude of each activity count has no bearing upon the rate-setting logic's processing of that count. An activity count which greatly exceeds the threshold setting is treated no differently than one that just barely exceeds the threshold. As a result, the pacing rate may be increased by the same amount whether the patient is engaged in vigorous or only moderate levels of physical activity.